Calculate And Compare
The specific heat of molding sand is roughly: 9.22×102J/kg·℃, the specific heat of water is: 4.19×103J/kg·℃, the heat of evaporation of water is: 2.26×106J/kg, add 20℃ water to 1 ton of sand 10kg (add 1% water) to raise the temperature to 50℃, the heat that can be taken away is 4.19×103×10×30, which is 12.57×105J. The temperature of 1 ton of sand is reduced by 1℃, and the heat dissipation is 9.22×102×1000 J, which is 9.22×105J. Therefore, adding 1% of water to the old sand can only reduce the temperature by 24.5°C. To evaporate 1% (10kg) of water in 1 ton of sand, the heat that can be taken away is 2.26×107J, but the sand temperature can be reduced by 24.5°C. The above analysis shows that: simply adding water to the belt conveyor or sprinkling water on the sand pile, the cooling effect is very poor. Even if water is added to the sand surface, it will not improve much. After adding water, the water should be evenly dispersed in the molding sand, and then the loose sand should be blown to make the water evaporate quickly while removing the steam. There are many types and specifications of sand cooling devices, mainly cooling drums, double-plate coolers and cooling boiling beds, etc., all of which use water to evaporate and cool the sand. Among them, the effect of cooling the boiling bed is better.
Moisture Control Of Old Sand
Almost all foundries inspect and control the moisture content of the mixed sand. However, the leaders and technicians of many foundries still lack sufficient understanding of the importance of strictly controlling the moisture content of the used sand. The moisture of the old sand entering the mixed sand is too low, and the impact on the quality of the sand may be as high as the sand temperature. Experimental research and experience have proved that it is much more difficult to wet dry bentonite with water than to lubricate wet bentonite. The bentonite and water in the molding sand are not simply mixed together, but they must be rubbed to make them into a plastic state. This is like making pottery with clay and water. The combination of water and soil is loose and has no bonding ability. After kneading and beating, each grain of soil can fully absorb water and become a plastic state. It can be shaped to make pottery blanks. After the casting of the mold, due to the influence of the hot metal, the soil-water bonding film on the surface of many sand grains is dehydrated and dried. It is not easy to add water to make it absorb water and restore its plasticity. The old sand has a lower moisture content, and the longer it takes to add water to the sand mixer to achieve the required performance. Since the time for sand mixing in production is limited, the lower the moisture content of the old sand, the worse the overall quality of the mixed sand. Foundry workers in various countries have reached a consensus: the moisture of the old sand entering the sand mixer can only be slightly lower than that of the mixed sand. A better approach is: the water contained in the old sand is slightly lower than that of the mixed sand after it is fully cooled by adding water during the cooling process. In this way, there is still a long time from when the sand is cooled to when it enters the sand mixer, and the water can fully wet the bentonite on the surface of the old sand grains. A better approach is to set up a sand mixer in the system to premix the old sand, and the cooled old sand is premixed with water in the premix sand mixer to improve the mixing state of bentonite and water in the old sand. In foreign countries, when premixing in some foundries, all additional materials such as new sand, bentonite, and coal powder that need to be added are added. Tianjin Xinweixiang Foundry Co., Ltd. used the EiRich sand mixer made in Germany for premixing. After the pre-mixed old sand enters the sand mixer, the amount of water added is small, only slightly adjusted. The bentonite and water in the molding sand are further modulated by the sand mixer, and the performance of the molding sand is more stable and consistent.
Granularity Of Old Sand
For iron castings made of clay green sand, the particle size of the sand should be finer. Since the amount of used sand during sand mixing is generally more than 90%, the main factor that determines the particle size of the molding sand is the used sand. The amount of new sand added is very small, and it is impossible to change the particle size of molding sand by adding new sand. Therefore, the particle size of the old sand should be checked frequently. When testing the particle size, clean and remove the sludge after sampling (the sand sample left when measuring the sludge content can be used), dry and then sieving.
The Content Of Water-Absorbing Fine Powder
The water-absorbing fine powder is mainly dead clay, but also includes coked coal fine particles and other fine powders. The content of water-absorbing fine powder is not as low as possible, and it is best to control it between 2-5%. The water-absorbing fine powder will compete with bentonite for water during sand mixing, so that the water required for the mixed sand to reach the target compactness value is increased. However, according to everyone’s understanding, the water absorption capacity of water-absorbing fine powder is stronger than that of bentonite, but its water retention capacity is lower than that of bentonite. Therefore, the amount of water added to the molding sand is slightly inappropriate, and the water-absorbing fine powder has a certain “fine-tuning and stabilizing” effect on the performance of the molding sand. When the water content is high, the fine powder first absorbs water, and the water absorbed by the bentonite can be more stable and consistent; when the water evaporates during the transportation process of the mixed sand, the water absorbed by the water-absorbing fine powder evaporates first, and the water in the clay paste that binds the sand is more stable. The performance is also less fluctuating. Too high content of water-absorbent fine powder is not good, it will make the moisture of the molding sand higher, and easily lead to the defects of pinholes, rough surface and sand holes on the castings. If the content of water-absorbent fine powder is too low, the properties of the molding sand (especially compactness) will not be easy to stabilize.